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Bragée, Petra
2013
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Bragée, P. (2013). A palaeolimnological study of the anthropogenic impact on dissolved organic carbon in South Swedish lakes. Centre for Environmental and Climate Research, Lund University.
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A palaeolimnological study of the anthropogenic impact on
dissolved organic carbon in South Swedish lakes
Petra Bragée
Avhandling
Att med tillstånd från Naturvetenskapliga Fakulteten vid Lunds Universitet för avläggande av filosofie doktorsexamen, offentligen försvaras i Geocentrum IIs föreläsningssal Pangea, Sölvegatan 12, fredagen den 1 november 2013 kl. 13.15.
Fakultetens opponent: Prof. Dr. Richard Bindler, Institutionen för ekologi, miljö och geovetenskap, Umeå Universitet, Umeå, Sverige
Lund 2013
Department of Geology & Centre for Environmental and Climate Research
Petra Bragée
A palaeolimnological study of the anthropogenic impact on dissolved organic carbon in South Swedish lakes
23 September, 2013
During the past three decades, increases have been observed in dissolved organic carbon (DOC) concentrations and colour in the surface waters of lakes and rivers in parts of Europe and North America, raising concern about the effects on the quality of aquatic environments with consequences for biodiversity, resource availability and recreational use. Various hypotheses have been put forward to explain the recent increases in DOC concentration and numerous studies have been published linking them to declining anthropogenic atmospherically deposited sulphur. Others have argued that increases in DOC content are a consequence of changes in climate, land use or land management practices.
The work presented in this thesis concentrates on identifying the major forcing mechanisms behind observed increases in DOC concentration in the upland area of southern Sweden during recent decades, by comparing variations in the total organic carbon (TOC) concentration in lake water inferred from lake sediments, in response to changes in land use, sulphur deposition and climate during the past eight centuries. Two small lakes with different catchment properties were selected for the study; one dominated by woodland with abundant peat deposits, and another located nearby with patches of agricultural land in an otherwise mainly forested terrain. A number of palaeolimnological methods were applied to the sediment sequences; decadal-scale variations in TOC concentration in the lakes were reconstructed based on visible-near infrared spectroscopy (VNIRS) of sediment successions, high-resolution (20-y) pollen-based reconstructions of local land use were quantified using the Landscape Reconstruction Algorithm (LRA) and the model Local Vegetation Estimates (LOVE), geochemical records provided further information on environmental changes in the lakes and their catchment areas, and changes in pH in the lakes were inferred from diatom analysis. Comparisons were made with population density data and climate records.
The results obtained with the LRA and LOVE models revealed a dynamic land-use pattern, with agricultural expansion from AD 1500 to the end of the 1800s, when population growth and the related increase in the exploitation of the surrounding land had a major impact on catchment erosion and input of terrestrial inorganic and organic matter to the lakes. Evidence also exists of a period of agricultural expansion around AD 1200-1300, followed by partial abandonment of the landscape, which can probably be attributed to the Black Death pandemic. A transition from traditional to modern land use following the industrial revolution took place during the past century, and a concurrent shift in most of the proxy records at around AD 1900 suggests a marked change in external forcing mechanisms common to both lakes, related to a major decrease in population density and the introduction of modern land use. The results revealed generally high TOC concentrations in the lakes prior to AD 1900, with second-order variations associated mainly with changes in the intensity of agricultural land use. The TOC concentrations in the lakes started to decrease around AD 1900, and unusually low TOC concentrations were recorded in the period AD 1930-1990, followed by a recent increase. The variation in sulphur emissions, with an increase in the early 1900s to a peak around AD 1980 followed by a significant decrease, was probably the main driver of lake-water TOC dynamics during the past century. This demonstrates that declining atmospheric sulphur deposition is the most probable driver of the increase in TOC concentration during the past three decades and that these lakes may be recovering to their naturally high-TOC pre-depositional states. The results also demonstrate regional versus local forcing of environmental change and indicate broadly similar regional sensitivities to anthropogenic impact, although responses were site-specific due to the different properties of the catchment areas. Given the reduction in atmospheric sulphur emission during recent decades, it is likely that previously suppressed or masked effects of changes in land use, land management and climate during the past century will become progressively more important drivers of TOC concentrations in lake water in the future. Long-term records of environmental history on decadal to millennial time scales enabled the assessment of ecosystem variability and responses to past anthropogenic disturbance, and may be a useful tool for the development of future environmental management strategies.
DOC, Brownification, Landscape Reconstruction Algorithm, Land-use changes, Lake sediments, Anthropogenic impact
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DOKUMENTDA
TABL
AD enl SIS 61 41 21
impact on dissolved organic carbon
in South Swedish lakes
Petra Bragée
Quaternary Sciences
Department of Geology
&
Centre for Environmental and Climate Research
Lund University
Sölvegatan 12, 223-62 Lund, Sweden
This thesis is based on the four papers listed below, which have been appended to the thesis. Paper I is in press in the journal indicated. Paper II, Paper III and Paper IV have been submitted to the indicated journals for consideration.
Paper I: Bragée P, Choudhary P, Routh J, Boyle JF, Hammarlund D (2013) Lake ecosystem responses to catchment disturbance and airborne pollution: an 800-year perspective in southern Sweden. Journal of Paleo-limnology (in press)
Paper II: Mazier F, Broström A, Bragée P, Fredh D, Stenberg L, Thiere G, Sugita S, Hammarlund D. Two hundred years of land-use change in the South Swedish Uplands: comparison of historical map-based estima-tes with pollen-based reconstruction using the Landscape Reconstruction Algorithm. Review of Palaeobotany and Palynology (in review)
Paper III: Bragée P, Mazier F, Rosén P, Fredh D, Broström A, Granéli W, Hammarlund D. Forcing mecha-nisms behind variations in total organic carbon (TOC) in lake water during the past eight centuries – Palaeo-limnological evidence from southern Sweden. Submitted to Biogeosciences
Paper IV: Fredh D, Mazier F, Bragée P, Lagerås P, Rundgren M, Hammarlund D, Broström A. The effect of local land-use change on floristic diversity around two lakes in southern Sweden, AD 1000-2008. Submitted to The Holocene
1. Introduction
1
1.1 Background and objectives
1
1.2 DOC content and colour of lake water
2
1.3 Long-term DOC trends in lake water
3
1.4 Brownification, regional trends
4
2. Summery of Papers
6
2.1 Paper I
6
2.2 Paper II
6
2.3 Paper III
6
2.4 Paper IV
8
3. Study Area and Lakes
8
3.1 Present conditions
8
3.2 Local history
9
3.3 Site-specific trends in lake-water DOC and colour
10
4. Methods - Reconstructing the Past
11
4.1 Stratigraphic approach
11
4.1.1 Sediment coring and subsampling
11
4.1.2 Dating and chronologies
11
4.2 Environmental reconstructions
13
4.2.1 Lake-water chemistry
13
4.2.2 Nutrient cycling and the origin of organic matter
13
4.2.3 Catchment-scale land use
15
4.2.4 Catchment-scale erosion
17
5. Results and Discussion
17
5.1 VNIRS-inferred TOC concentrations, colour and long-term trends in
Åbodasjön and Lindhultsgöl
17
5.2 Identifying anthropogenic and climate impact on DOC
19
5.2.1 Land use
19
5.2.2 Sulphur deposition
21
5.2.3 Climate
23
5.3 Future developments
25
6. Conclusions
26
Acknowledgements
27
Svensk sammanfattning
28
References
30
1. Introduction
1.1 Background and objectives
During the past thirty years, increases in dissolved organic carbon (DOC) concentrations and the colour of lakes and running water have been observed in many areas, including Scandinavia (Hongve et al. 2004; Skjelkvåle et al. 2005; Vuorenmaa et al. 2006; Erlandsson et al. 2008; 2010; Sarkkola et al. 2009; Arvola et al. 2010), the Baltic states (Pärn and Mander 2012), the UK (Evans et al. 2005; Worrall and Burt 2007; Chapman et al. 2010), Central Europe (Hejzlar et al. 2003; Oulehle and Hruška 2009) and parts of North America (Stoddard et al. 2003; Findlay 2005; SanClements et al. 2012). This has led to concern regarding the effects on the function of the aquatic environments with consequences for biodiversity, resource availability and recreational use. DOC is operationally defined as the fraction of organic matter that passes through a filter with a nominal pore size of 0.45 μm (Wetzel 2001). Due to the dark colour of many DOC compounds, DOC affects the light regime, thermal structure and mixing depth of lakes, affecting the aquatic primary and secondary production (Cole et al. 2007; Karlsson et al. 2009; von Einem and Granéli, 2010). DOC can also cause acidification of surface waters (Kortelainen 1995) and influence the transport of trace metals and organic contaminants (Lawlor and Tipping 2003). Furthermore, the export of DOC from the terrestrial and aquatic ecosystems to the coasts constitutes an important part of global carbon cycling through local landscapes and continents (Tranvik et al. 2009),
and contributes to CO2 emissions to the atmosphere
(Cole et al. 2007; Weyhenmeyer et al. 2012). Concern has also been raised about drinking water quality, as increasing DOC concentrations can enhance bacterial growth and the transport of contaminants and toxic compounds with potential carcinogenic and mutagenic properties (Ledesma et al. 2012). DOC can also affect recreational activities, such as fishing and swimming. There is general concern that an increase in DOC constitutes a hazard to the environment, and methods of controlling or preventing this trend must be found.
In Sweden, the observed increase in DOC content and thus colour in lakes, rivers and coastal waters, also referred to as “brownification” (Granéli 2012), has recently been recognized as an environmental problem and a future challenge (Swedish Agency for Marine and Water Management).
Various hypotheses have been put forward to explain the recent increases in DOC concentration (see Evans et al. 2005; Porcal et al. 2009), and numerous studies have been published linking them to declining anthropogenic atmospherically
deposited sulphur (SO42-) (Evans et al. 2005; 2006;
2012; Vuorenmaa et al. 2006; Monteith et al. 2007; Erlandsson et al. 2008; Oulehle and Hruška 2009; Arvola et al. 2010; Clark et al. 2010; Ekström et al. 2011; SanClements et al. 2012). Others have argued that increases in DOC content are a consequence of changes in climate (Freeman et al. 2001; Hudson et al. 2003; Hejzlar et al. 2003; Hongve et al. 2004; Worrall and Burt 2007; Eimers et al. 2008; Lepistö et al. 2008; Sarkkola et al. 2009). In other studies, elevated DOC content has been linked to land use and land management practices, such as nitrogen deposition and fertilization (Findlay 2005; Correll et al. 2001), heather burning (Yallop et al. 2010) and drainage (Armstrong et al. 2010). However, there is no overall scientific consensus on the mechanisms controlling the observed variations in DOC and colour in lakes over recent decades. This suggests that individual lakes respond differently to the suggested forcing mechanisms, and that brownification can not be ascribed to a single forcing mechanism.
Previous studies have been based on experiments or data covering a few decades, and provide only a snapshot of the short-term and present-day conditions. There is, therefore, a lack of information on the natural variability and temporal evolution of DOC on the centennial and millennial scales, i.e. beyond monitoring data. Many of the suggested drivers of brownification are caused by human activities during the past century, and it is necessary to adopt a long-term perspective to distinguish between anthropogenic and natural changes. Palaeolimnology and lake sediment analysis can provide unique information on the environmental history of lacustrine ecosystems and their terrestrial
surroundings, providing evidence of the nature and timing of environmental change. This enables the causes of change to be identified in many cases. Such information is invaluable for projections of future ecological trends and the development of strategies for environmental management (Renberg et al. 2009). Palaeolimnological studies have successfully helped identify the effects of anthropogenic impacts on the lacustrine ecosystem, for example, eutrophication (Fritz 1989; Bradshaw et al. 2005), pollution (Renberg et al. 2000; Bindler et al. 2008) and acidification (Renberg 1990a).
This project was initiated to explore possible forcing mechanisms behind the observed brownification and increase in DOC concentration in lakes by comparing the impact of changes in land use, sulphur deposition and climate on long-term (centennial to millennial) variations in the total organic carbon (TOC) concentration in lake water. The TOC content of Scandinavian surface waters in lakes is dominated by DOC (>95%) (Mattsson et al. 2005), the remaining fraction being particulate organic carbon (POC). A number of palaeolimnological methods were applied to sediment sequences, including reconstruction of TOC concentration based on visible-near-infrared spectroscopy (VNIRS), diatom analysis to determine water pH, and pollen analysis to determine catchment land cover using the Landscape Reconstruction Algorithm (LRA), and geochemistry. Two lakes, Åbodasjön and Lindhultsgöl, in southern Sweden were chosen for this study because of their observed increase in TOC content in recently deposited sediments, in spite of their different catchment properties. Comparison of companion sediment records in different settings will allow the identification of regional and local forcing mechanisms of TOC variations. A third lake, Fiolen, was selected to reconstruct the regional vegetation, which was used in the pollen-based LRA to quantify local vegetation and land use, in a joint project investigating the impact of past changes in land use on floristic diversity.
The aims of this study were:
• to explore possible forcing mechanisms
behind the observed present-day increase in DOC content in two southern Sweden lakes by comparing
changes in land use, acid deposition and climate with long-term reconstructions of TOC concentration inferred from lake sediments (Paper III),
• to quantify long-term changes in land use
on a local scale using the LRA based on fossil pollen records and comparison with historical maps to assess the catchment scale forcing on TOC variations (Papers II and IV),
• to assess how the aquatic ecosystems of two
lakes with contrasting catchment characteristics have been affected by anthropogenic activities and climate change during the past 800 years (Papers I and III), and
• to introduce the potential future change in
lake-water TOC concentration as an important variable in land management and policy decisions affecting drinking water (Paper III).
1.2 DOC content and colour of lake water
The DOC content in boreal forested catchment areas typically originates from organic breakdown of plant material and the leaching of decayed dead organisms from terrestrial soils. A minor fraction of the DOC pool arises from aquatic sources such as leachate from dead organisms, phytoplankton and aquatic macrophytes (Bade et al. 2007). DOC can be divided into a humic fraction, consisting of organic compounds of high molecular weight and refractory organic matter, generally characterized as being yellow to black in colour, and a colourless non-humic fraction, consisting, for example, of lipids, carbohydrates and amino acids. Humic substances are ubiquitous in water, soil and sediments, and usually comprise 50-75% of the DOC in water, but may exceed 95% in very coloured lakes (McDonald et al. 2004).As humic substances constitute a major part of the DOC, DOC concentrations in lakes are usually strongly correlated to water colour (Pace and Cole 2002; von Einem and Granéli 2010). However, some studies have reported clear discrepancies between DOC concentrations and the colour of lake water (Erlandsson et al. 2008; Kritzberg and Ekström 2012), indicating that the composition of DOC at the molecular level may be equally important for changes in water colour. Moreover,
iron (Fe) and manganese (Mn) also have a strong influence on water colour (Maloney et al. 2005). Elevated iron concentrations have been observed during the past few decades in the UK (Neal et al. 2008), as well as in Sweden (Huser et al. 2011; Kritzberg and Ekström 2012). Kritzberg and Ekström et al. (2012) found a strong correlation between water colour and iron concentration, indicating that brownification of some waters in Sweden was more likely to be related to increases in iron than organic carbon.
The amount of DOC leaching from terrestrial soils into lakes is driven by factors affecting the production, solubility and transport of DOC (see Clark et al. 2010). Biological processes involved in organic production and decomposition are controlled by factors affecting plant growth and soil organisms (i.e. climate and nutrient availability). Chemical processes regulate the solubility of organic carbon through pH and the ionic strength of the soil solution, while precipitation and hydrology control the export of DOC to surface waters via runoff and water flow in soils. Hence, the composition and quantity of DOC may differ between sites depending on climate and catchment properties such as vegetation, hydrology and soil properties, which may be altered by natural or human-induced changes in conditions. The DOC concentration is further regulated by gradual degradation by mineralization, photooxidation and sedimentation within the lake (Tranvik et al. 2009). In boreal lakes, this degradation results in a loss of
carbon through sedimentation and CO2 emission
to the atmosphere, which have similar magnitudes (von Wachenfeldt and Tranvik 2008).
Water colour is usually determined by measurements of absorbance at wavelengths around 400 nm, or by using a platinum salt solution as
reference, giving the colour as mg Pt L-1. Water
colour can also be assessed by DOC and TOC concentrations (Table 1). DOC concentrations in lake water are dynamic and show seasonal and long-term variations over several years or decades (see Clark et al. 2010). Generally, DOC concentrations
in freshwater vary from 1 to 30 mg L-1 and are
highest in lakes from forested and peaty catchment areas with soils associated with high organic matter content, i.e., nutrient-poor systems, where organic production is greater than decomposition (Aitkenhead and McDowell 2000). The TOC content of Scandinavian lakes ranges from very low
(< 1 mg L-1) in the mountainous regions, to above
20 mg L-1 in Finland and the south-east of Sweden.
1.3 Long-term DOC trends in lake water
Recent changes in DOC concentrations have raised questions about the natural variability and temporal evolution of DOC on centennial and millennial scales. According to the European Union Water Framework Directive (EU WFD) (European Union 2000) all member countries are obliged to achieve “good“ water status for relevant water bodies by AD 2015, defined as “conditions deviating only slightly from undisturbed conditions with low levels of anthropogenic disturbance”. To achieve this, it is important to understand the reference conditions for DOC in lake water, i.e., the “background conditions with no, or minimal anthropogenic stress” (Bennion et al. 2004). The date for reference conditions has generally been set to around AD 1850 (Bennion et al. 2004; Bjerring et al. 2008; Erlandsson et al. 2011), immediately before the onset of industrialisation. However, this date has been debated since different countries, regions and individual catchment areas often have different Table 1. Classification of colour, absorbance and total organic carbon (TOC) in lakes and running water (Swedish Environmental Protection Agency)Colour (mg Pt L-1) Absorbance (filtered) TOC (mg L-1)
Not or slightly coloured < 10 < 0.02 < 4
Slightly coloured 10-25 0.02-0.05 4-8
Moderately coloured 25-60 0.05-0.12 8-12
Significantly coloured 60-100 0.12-0.20 12-16
histories regarding human influence (Bennion et al. 2011). It has been shown in palaeolimnological studies that Swedish lakes have been significantly affected by anthropogenic activities in pre-industrial times, about 4000 years ago, through settlements, agriculture and pollution (Renberg et al. 2000: Berglund et al. 2002). It may sometimes be difficult to differentiate between natural and anthropogenic changes to the lake ecosystem, as several processes could be due to natural or anthropogenic forcing. For example, sediment evidence of eutrophication may be identical to anthropogenic nutrient pollution (Jeppesen et al. 2010).
Available long-term DOC records are based on either VNIRS-inferred TOC reconstructions (Rosén 2005) or diatom-based calibration models providing quantitative measures of the DOC content in lake water (Kingston et al. 1990). The VNIRS approach has recently gained increasing attention as a proxy for ambient variations in DOC concentrations in lake water. Compared to the diatom-based approach, VNIRS models generally show better statistical performance, as diatoms respond to several environmental variables and may be influenced by a stronger pH signal.
A long-term study on four lakes in southern Sweden, covering about 300 years, revealed considerable decreases in VNIRS-inferred TOC concentrations in lake water over the past two centuries (Cunningham et al., 2010), in contrast to the reported present-day high DOC contents. On an even longer timescale (12,500 BP), Rosén et al. (2011) reported predominantly higher VNIRS-inferred TOC during most of the Holocene in a south Swedish lake and decreasing TOC concentrations during the past millennium. This is similar to findings in other long-term studies in boreal forest and subarctic lakes in northern Sweden (Rosén, 2005; Rosén and Hammarlund, 2007; Reuss et al., 2010). Rosén et al. (2011) suggested that the decrease in TOC content was caused by reduced transport of organic carbon from the catchment area due to early anthropogenic activities including agriculture, and man-made fires for clearance, resulting in more open land with lower biomass production. Limited soil carbon pools were also used to explain initial low TOC concentrations during early soil and vegetation development
following deglaciation. Variations in long-term TOC content recorded in more pristine areas of northern Sweden with little or no anthropogenic impact have been explained by changes in climate, especially humidity, mire development and fire frequency (Rosén and Hammarlund 2007). In a study carried out in North America, decreasing diatom-inferred DOC concentrations were observed concurrently with decreasing pH in some recently acidified lakes (Kingston et al. 1990).
These studies suggest that the “reference level” of DOC in lake water was generally higher than today, and that the observed decrease over the past millennium and centuries in southern Sweden may be due to anthropogenic activities. This clearly illustrates the importance of adopting a long-term perspective in our attempts to understand the historical development of DOC levels, and to achieve conditions in lakes close to undisturbed conditions according to the EU WFD (European Union 2000).
1.4 Brownification, regional trends
Both the lakes studied in the present work are situated in the part of Sweden subject to the most significant increases in TOC concentrations over the period AD 1990-1999, according to monitoring data from 344 lakes in Scandinavia (Löfgren et al. 2003). Long-term individual time series from southern Sweden starting earliest AD 1960 (http:// miljodata.slu.se/mvm/) show some variability in colour and TOC concentration patterns in lakes (Fig. 1). Generally, most of the lakes showed an increasing trend in colour and TOC concentration since the beginning of the measurements. However, some of the lakes showed no distinct trends in colour during the period, notably Nässjön and V. Skälsjön. One lake, Farstusjön, showed decreasing trends from AD 2000. The most significant changes were observed in the lakes with the highest baseline colour, a relationship observed by others (Worall and Burt 2004a). Generally, lake colour and TOC content exhibit the same trends, with a few exceptions, for example, Gyslättasjön and Skärlen.
A survey of 38 lakes in the southern Swedish uplands, within an area of approximately 150 x 100 km, was conducted in July 2007, and included the
two lakes studied in the present work (von Einem and Granéli 2010). It was found that at least half of the lakes showed a significant increase in colour after AD 1990, while other lakes showed no significant changes, or even a decrease in colour (Fig. 2). Lakes within the same region usually exhibit similar physical, chemical and biological characteristics, and the results suggests there is a possible common regional driving mechanism to the observed increases in DOC concentrations of the majority of the lakes in this area. But these results also suggest that there may be differences in the driving mechanisms of recent brownification, and that variations in the characteristics of the catchment area may lead to different DOC concentrations in individual lakes.
100 km 10. 6. 5. 4. 3. 2. 1. 8. 12. 7. 9. 11. 7. 200 100 0 250 150 50 5 10 15 20 25 30 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 3. 100 0 50 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 4. 0 50 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 11. 1960 1970 1980 1990 2000 2010 0 10 20 30 40 50 10 5 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 6. 200 100 0 150 50 5 10 15 20 25 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 9. 1960 1970 1980 1990 2000 2010 400 300 200 100 0 350 250 150 50 5 10 15 20 25 30 35 40 45 50 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 1. 5 10 50 0 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 8. 50 0 5 10 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 10. 100 0 150 50 5 10 15 20 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 450500 12. 550 600 55 60 65 70 400 300 200 100 0 350 250 150 50 5 10 15 20 25 30 35 40 45 50 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 2. 100 0 50 5 10 15 1960 1970 1980 1990 2000 2010 W at er c olour (mg P t L ) -1 TOC (mg L ) -1 5. 5 10 15 20 1960 1970 1980 1990 2000 2010 100 0 150 50 W at er c olour (mg P t L ) -1 TOC (mg L ) -1
Figure 1. Long-term monitoring data from twelve lakes in southern Sweden. the map show the location of the lakes. Lake-water
colour (mg Pt L-1) is shown by the black curve and total organic carbon (TOC) concentration (mg L-1) by the grey curve for
each lake, as a function of time. Measurements of water colour were generally replaced by measurements of absorbance (400 nm) in the AD 1980-1990s, and have been converted here to water colour by multiplying by 500, according to the standard method (Swedish Environmental Protection Agency, 1999). The vertical lines in each diagram indicate the year AD 1990. The shaded areas represent different colour regimes in each lake according to the classification in Table 1. Light brown: not or slightly coloured, medium brown: moderately coloured and dark brown: significantly or highly coloured water.
1. V. Skälsjön 2. Lien 3. Stensjön 4. St Härsjön 5. Stengårdhultasjön 6. Nässjön 7. Gyltigesjön 8. Skärlen 9. Vrången 10. Gyslättasjön 11. Fiolen 12. Farstusjön 250 200 150 100 50 0 0 5 10 15 20 25 30 35 40 W at er c olour (mg P t L -1) Number of lakes Water colour in AD 1990 Water colour in AD 2007
Figure 2. Data from 38 lakes sampled in July 2007 (von Einem and Granéli 2010) in the southern Swedish uplands, showing the change in water water colour since AD1990.
2. Summary of Papers
2.1 Paper I
Bragée P, Choudhary P, Routh J, Boyle JF, Hammar-lund D (in press) Lake ecosystem responses to catchment disturbance and airborne pollution: an 800-year per-spective in southern Sweden. Journal of Paleolimno-logy
Variations in nutrient cycling and deposition of lithogenic elements were studied in response to anthropogenic impact and catchment disturbance in two small lakes in the south Swedish uplands, Åbodasjön and Lindhultsgöl, during the past eight centuries. Variations in lake sediment carbon, elemental nitrogen and isotopic ratios (TOC, TN,
δ13C and δ15N) were studied together with n-alkane
organic geochemistry, to characterize nutrient cycling dynamics, and to distinguish terrestrial and aquatic sources of lacustrine organic matter. Changes in fluvial and airborne delivery of inorganic elements (K, Ti, Rb, Zr, P, Pb and Zn) to the lakes were determined using X-ray fluorescence. The results revealed that population growth and related increases in land-use pressure had a major impact on catchment erosion and thus the input of terrestrial organic matter to the lakes from the 1500s to the end of the 1800s. Evidence was also found of a brief period of catchment disturbance about AD 1200-1300, followed by recovery, probably due to the Black Death pandemic. In about AD 1900 synchronous shifts in most of the proxy records suggest a marked change in external forcing mechanisms common to both lakes, related to a major decrease in population density and the introduction of modern land use following the industrial revolution. The results also demonstrated differences in regional and local forcing of lake ecosystems, but indicated broadly similar sensitivities of the lake ecosystems to human impact, with site-specific responses to local disturbances during the past century due to different nutrient conditions and catchment area properties.
2.2 Paper II
Mazier F, Broström A, Bragée P, Fredh D, Stenberg L, Thiere G, Sugita S, Hammarlund D. (in review) Two
hundred years of land-use change in the South Swedish Uplands: comparison of historical map-based estimates with pollen-based reconstruction using the Landscape Reconstruction Algorithm. Review of Palaeobotany and Palynology
The local variation in land use in the South Swedish Uplands over the past 200 years, based on pollen records from three lake-sediment successions, was studied to assess ecosystem variability and responses to past anthropogenic disturbances. Temporal changes in the proportional cover of 14 plant taxa were quantified as percentages using the Landscape Reconstruction Algorithm (LRA). The LRA-based estimates of the extent of four categories of land use (cropland, meadows/grassland, wetland and outland/woodland) were compared to corresponding estimates based on historical maps and aerial photographs from AD 1769-1823, AD 1837-1895, AD 1946 and AD 2005. The LRA-reconstructed vegetation composition is generally in good agreement with estimates based on the historical records. The LRA approach was used to reconstruct the 200-year history of local land-use dynamics at 20-year intervals around two small lakes, Åbodasjön and Lindhultsgöl. The results showed differences in historical land use between the sites, and that local catchment characteristics, such as soil conditions and wetland cover, also appeared to be important for the development of human impact. Hence, the application of the LRA approach with high-resolution pollen records can provide detailed information on land-use dynamics on decadal to millennial timescales, providing the potential to evaluate the impact of land use on terrestrial and aquatic ecosystems, and land-use dynamics should, therefore, be taken into account when nature conservation strategies are being developed.
2.3 Paper III
Bragée P, Mazier F, Rosén P, Fredh D, Broström A, Granéli W, Hammarlund D. Forcing mechanisms be-hind variations in total organic carbon (TOC) in lake water during the past eight centuries – Palaeolimno-logical evidence from southern Sweden. Submitted to Biogeosciences
forcing mechanisms behind observed increases in lake-water dissolved organic carbon concentration (DOC) in the upland area of southern Sweden during recent decades, by comparing the impacts of changes in land use, sulphur deposition and climate to long-term trends in total organic carbon (TOC) concentration in lake water since AD 1200 inferred from proxy records from two small lakes, Åbodasjön and Lindhultsgöl. Decadal-scale variations in TOC concentration in lake water were reconstructed based on visible-near infrared spectroscopy (VNIRS) of sediment successions. Changes in pH were inferred from diatom analysis. Pollen-based reconstructions of local land use, quantified using the Landscape Reconstruction Algorithm (LRA) and geochemical records, provided information on
catchment-scale environmental changes, and comparisons were made with records of climate and population density. The results revealed generally high TOC concentrations in lake water prior to AD 1900, with second-order variations associated mainly to changes in the intensity of agricultural land use. Significant changes were found in the past century, and unusually low TOC concentrations were recorded in AD 1930-1990, followed by a recent increase. The variation in sulphur deposition, with an increase in the early 1900s leading to a peak around AD 1980, followed by a decrease, was most likely the main driver of these dynamics. Declining atmospheric sulphur emission is the most probable driver of the increase in TOC concentration in lake water during the past thirty years. This suggests that Table 2. Authors contributions to Papers I-IV
Paper I Paper II Paper III Paper IV
Fieldwork P. Bragée F. Mazier A. Broström D. Hammarlund P. Bragée F. Mazier A. Broström D. Hammarlund P. Bragée F. Mazier A. Broström D. Hammarlund P. Bragée F. Mazier A. Broström D. Hammarlund Core correlation and sample
preparation P. Bragée P. Bragée P. Bragée P. Bragée
Age-depth model P. Bragée
D. Hammarlund P. Bragée D. Fredh D. Hammarlund
P. Bragée
D. Hammarlund P. Bragée D. Fredh D. Hammarlund
XRF analysis P. Bragée P. Bragée P. Bragée P. Bragée
C and N elemental and stable
isotope analysis P. Bragée P. Bragée
n-alkane analysis P. Choudhary
J. Routh
VNIRS analysis P. Rosén
Diatom analysis P. Bragée
Pollen analysis F. Mazier
D. Fredh F. MazierD. Fredh F. MazierD. Fredh
LRA modeling F. Mazier D. Fredh
F. Mazier D. Fredh
Historical maps and aerial
photo-graphs A. BroströmL. Stenberg
D. Fredh G. Thiere
Data interpretation P. Bragée
P. Choudhary J. Routh J.F. Boyle D. Hammarlund F. Mazier A. Broström P. Bragée D. Fredh S. Sugita D. Hammarlund P. Bragée F. Mazier D. Fredh P. Rosén W. Granéli D. Hammarlund A. Broström D. Fredh A. Broström F. Mazier P. Lagerås M. Rundgren P. Bragée D. Hammarlund
the lakes studied are recovering towards their natural high-TOC pre-depositional states. Given that the effects of sulphate deposition are declining, other forcing mechanisms related to land use and land management practices, and climate change will possibly become the main drivers of future changes in TOC concentration in the two studied lakes.
2.4 Paper IV
Fredh D, Mazier F, Bragée P, Lagerås P, Rundgren M, Hammarlund D, Broström A. The effect of local land-use change on floristic diversity around two lakes in southern Sweden, AD 1000-2008. Submitted to The Holocene
The relationship between land use and floristic diversity was analysed around two lakes in southern Sweden, Åbodasjön and Lindhultsgöl, over the past thousand years. Pollen analysis and the Landscape Reconstruction Algorithm approach including the Local Vegetation Estimates (LOVE) model were used to quantify land cover on local scales with high resolution (20 to 50 years). Floristic richness was estimated using palynological richness, and LOVE-based evenness was introduced as a proxy for floristic evenness on a local scale based on the LOVE output. The results revealed a dynamic land-use pattern, with agricultural expansion during the 1200s, partly abandoned landscape around AD 1400, re-establishment during the 1400-1500s and a transition from traditional to modern land use during the 1900s. The results suggest that the more variable landscape and dynamic land use during the 1200s to 1800s characteristic of the traditional landscape were of substantial importance in achieving high floristic diversity. The landscape provided different production services, such as grazing areas for cattle, and regulating services, such as pollination of crops and invasion resistance. During approximately the past 100 years, areas with high floristic and faunal diversity such as meadows and pastures have decreased in favour of crop cultivation and timber production, i.e. potential areas for providing cultural and regulating services. More habitats are also related to coniferous woodlands, and fewer habitats are related to deciduous trees and open land taxa, which may not be sustainable in preserving floristic diversity in the
future. Palynological richness sometimes remains at higher levels during periods of regression, at least during roughly the first 40 years, which suggests that many plants can survive during periods of succession and reforestation. The variability in past agricultural landscape provides information about types of land use that promote floristic diversity, which is potentially useful for nature conservation and the implementation of the framework of ecosystem services.
3. Study Area and Lakes
3.1 Present conditions
The location of the area studied is illustrated in Figure 3. The crystalline bedrock in the area studied is dominated by granite and gneiss (Wikman 2000), and is covered by sandy glacial till of various thicknesses, with occasional glaciofluvial deposits and scattered peat deposits (Daniel 2009). The climate is generally maritime with a mean annual temperature of 6.4°C (January –2.7°C, July 15.9°C) and an annual precipitation of 651 mm (January 52 mm, July 75 mm), based on reference values from the largest city in the region, Växjö, for AD 1961-1990 (Alexandersson et al. 1991). The regional vegetation belongs to the boreo-nemoral vegetation zone, and is characterized by woodland containing a mixture of coniferous and deciduous
100 km
Sweden
N
.
Figure 3. Map of Scandinavia and southern Sweden showing the study area (indicated by the square) and the closest city,
trees (Sjörs 1963; Gustafsson 1996).
The two lakes, Åbodasjön (221 m asl) and Lindhultsgöl (212 m asl), are situated in the upland area of southern Sweden, in a rural setting in the province of Småland. Åbodasjön is an oligotrophic mesohumic lake fed by two inlet streams, from the south and north-east, and has an outlet in the south-west. The village of Åboda (40 residents in 2004) is situated west of the lake, and the catchment area is currently dominated by managed coniferous woodland with semi-open areas of deciduous trees and cropland near the lake margins. Lindhultsgöl is an oligotrophic polyhumic lake with at least two artificial ditches draining into it from nearby wetland and woodland, and there is an outlet consisting of an artificial ditch in the south. The catchment area consists mostly of managed coniferous woodland and wetland with shrubs and pine trees. The morphometric and hydrological characteristics are given in Table 3, and detailed maps of the two lakes are illustrated in Figure 4.
3.2 Local history
The lakes are situated in Slätthög Parish (Fig. 4), an elongated area (22 km in length) covering about
138 km2, established in around AD 1000.
Archaeological remains in the area reveal human presence at least during the last 6000 years. The first local population data are available in church records from AD 1571, and show that the parish had 301
inhabitants (<3 inhabitants km-2; Andersson Palm
2000). During the 1700s, the population started to increase rapidly, and reached a peak of 2485 in AD
1865 (about 20 inhabitants km-2), followed by a
decrease in rural population in response to industrialisation in the late 1800s.
The general land-use history of the southern Swedish uplands is known through palaeo-ecological studies, archaeological evidence and historical records. The area has been subject to a number of agricultural expansions and regressions during the past 6000 years (Lagerås 1996; Berglund et al. 2002). The early expansions were dominated by grazing animals and single farms with small-scale agriculture (Berglund et al. 2002). This was followed by a general agricultural development in the uplands of southern Sweden in Mediaeval times (about AD
900-1200) towards permanent farming with permanent cropland and meadows, used for hay production, located close to the settlements. The land outside this centre, the outland, was the common land used for cattle grazing and the collection of firewood and timber. A second expansion took place in the 1500s, and records showed that 49 farms were established in the parish in the mid-1500s, and that farms were situated within 1-2 km of the two lakes studied in this work. Agriculture developed during the agricultural revolution (about AD 1700-1900) leading to improvements in yield from small-scale agriculture through better management of manure, crop rotation and marling (Emanuelsson 2009; Myrdal and Morell 2011). The maximum degree of agricultural land use was seen in the late 1800s, and during the past century small-scale agriculture has been replaced by modern land use dominated by commercial forestry and crop cultivation (Antonsson and Jansson 2011).
Historically, important local industries in the study area have included iron production based on bog ore (AD 1530-1610), potash manufacturing based mainly on beech wood (AD 1753-1800), the production of tar and charcoal based mainly on pine wood (AD 1600-1850), and nitrate production, based on manure, for gunpowder (AD 1780-1817). Gamleboda mill, situated at the outlet of Åbodasjön from AD 1756 and onwards, has
Åbodasjön Lindhultsgöl
Altitude (m) 221 212
Lake surface area (km2) 0.5 0.07
Maximum depth (m) 9 5
Catchment area (km2) 9.5 0.6
Residence time (years) 0.5
-pH 7.0 6.4 Alkalinity 0.56 0.83 Chlorophyll a conc.(μg L-1) 7.7 14.9 DOC conc.(mg L-1) 11.0 23.8 Water colour (mg Pt L-1) 40 960 Liming started 1984 1993
Table 3. Morphometric and hydrological characteristics of the two lakes studied, sampled in July 2007 (von Einem and Granéli 2010)
regulated the water level of the lake. Several sawmills were established in the study area in the 1950s.
3.3 Site-specific trends in lake-water DOC
and colour
In 2007, Åbodasjön was moderately coloured
(40 mg Pt L-1) with a DOC concentration of 11 mg
L-1. Lindhultsgöl was highly coloured (960 mg Pt
L-1) and had a DOC concentration of 23.8 mg L-1
(Table 1). Yearly monitoring data from the inlet of Åbodasjön, based on an average of several samples (County Administrative Board of Kronoberg, unpublished data), also showed an increasing trend
in water colour since the 1990s with an R2 value of
0.5188 (Fig. 5). Based on the monitoring data available, it can be concluded that Åbodasjön has been subject to brownification during the past three decades. Unfortunately, no monitoring data are available for Lindhultsgöl.
Figure 4. (a) Map showing the location of the lakes Åbodasjön, Lindhultsgöl and Fiolen. Modern land-use properties, catchment areas for each lake (red dashed lines), relevant source area of pollen (RSAP) (red circles), and the parish boundary (black dashed line) are shown. (b) Maps of the studied lakes showing modern land-use properties, catchment areas for each lake and important infrastructure. N Fiolen Slätthög Lake catchment Parish boundary Predicted RSAP Church Open land Woodland Wetland Water Lindhultsgöl Åbodasjön V V V Åbodasjön V Lindhultsgöl 0 1 2 km 0 1 2 3 4 5 km Road Buildings
b)
a)
4. Methods - Reconstructing the Past
4.1 Stratigraphic approach
4.1.1 Sediment coring and subsampling
Sediment cores were collected at the central parts of the lakes in spring 2008, at a water depth of 8.6 m in Åbodasjön and 5.2 m in Lindhultsgöl. Surface sediments sequences, 32 cm in Åbodasjön and 30 cm in Lindhultsgöl, were obtained at both sites using a gravity corer (HTH-Kajak corer: Renberg and Hansson 2008) from a platform, and divided in the field into 5-mm contiguous intervals. Overlapping sediment cores of 4.5 m in Åbodasjön and 2.6 m in Lindhultsgöl, were collected using 1-m-long Russian peat corers. All cores were transferred to supportive plastic liners, wrapped in plastic and transported to Lund, where the uppermost 1-m from each lake was divided into 5-mm contiguous sections in the laboratory. All sediment samples were kept in cold storage (4°C) prior to analyses. Correlations between the surface sediments and Russian core segments were based on mineral magnetic properties (Thompson 1980) measured at the Palaeomagnetic and Mineral Magnetic Laboratory at Lund University, and X-ray fluorescence measurements of element compositions (Boyle 2000).4.1.2 Dating and chronologies
Accurate dating of sediment cores and the construction of robust chronologies with high temporal resolution is necessary for the comparison of data collected at the two lakes and to ensure reliable interpretations of events. For this purpose, a combination of different dating methods was used.
For dating of the most recently deposited
sediments (e.g. <150 y), a lead radioisotope (210Pb)
was used to date sediment samples from the gravity cores (Appleby 2002). Sediment samples were
analysed to determine the activities of 210Pb, 226Ra
and 137Cs using gamma spectrometry at the Gamma
Dating Center, Institute of Geography at the University of Copenhagen, Denmark. The total
signal of 210Pb in the sediments consists of supported
210Pb from autogenic material and unsupported
210Pb originating from atmospheric deposition. The
unsupported 210Pb activity is the fraction used for
dating, and was calculated using the 226Ra
concentration, based on the assumption that
supported 210Pb is in equilibrium with its parent
nuclide 226Ra. The 210Pb decays with a half-life of 22
years, and the remaining amount of unsupported
210Pb activity left at a certain depth reveals the age
of the sediment. To calculate sediment ages, the constant rate of supply (CRS) model (Appleby 2002) was applied to the profiles. Variations in the
radioisotope 137Cs was used as time markers based
on peak concentrations associated with stratospheric testing of atomic weapons beginning in AD 1952, and the Chernobyl nuclear power plant accident in AD 1986.
For older sedimentary sequences, the most
common method of choice is radiocarbon (14C)
dating (Björck and Wohlfarth 2002). Radiocarbon is produced in the atmosphere by cosmic rays, and is taken up by all living organic matter on earth.
When an organism dies, 14C is no longer absorbed
and the remaining radiocarbon decays with a half-life of 5730 years. Selected macrofossil remains and bulk samples from both lakes were subjected to
accelerator mass spectrometry 14C dating at the
Radiocarbon Dating Laboratory, Lund University. Bulk samples can be subjected to “old carbon
Years (AD) 1980 1990 2000 2010 0 50 100 150 200 250 300 350 W at er c olour (mg P t L ) -1
Figure 5. Water colour in Åbodasjön, based on yearly mean values from monitoring data plotted against time, fitted
with linear regression (R2: 0.5188). The brown shaded areas
effects”, i.e. the incorporation of older carbon due to reworking of the sediment, resulting in higher ages (Barnekow et al. 1998; Reuss et al. 2010). Therefore, only dated macrofossils were used to determine the chronology. The radiocarbon age of each sample is not equivalent to calendar years, due
to natural variations in 14C production. All 14C
dates were converted into calendar years using the IntCal09 radiocarbon calibration dataset (Reimer et al. 2009) and the calibration program OxCal4.1 (Bronk Ramsey 2009).
The concentration of Pb in sediment can be used as a time marker, based on the regionally coherent pattern of Pb deposition resulting from airborne pollution over Sweden during recent millennia (Brännvall et al. 2001; Renberg et al. 2001). The large-scale trends include a substantial increase in Pb deposition around AD 1000, mainly related to mining and metal production in Europe, which reached a peak in about AD 1200. This was followed by a minimum around AD 1350 in response to plagues and recession, and another peak around AD 1530, reflecting increased silver ore processing in continental Europe (Brännvall et al. 2001). These four marker horizons were all clearly
seen in the Åbodasjön and Lindhultsgöl Pb records. Subsequent fluctuations at relatively high levels from about AD 1530 to 1900, followed by rising values to maximal Pb concentrations in the 1970s, due to the extensive use of leaded petrol, were also recorded. The 1970s Pb peaks were not incorporated
into the age models as robust 210Pb data are available
in this part of the sediment records. The Lindhultsgöl record also includes a minor peak in Pb deeper in the sediments, which is believed to represent the Greek and Roman civilizations around BC/AD 0 (Brännvall et al. 2001). The concentrations of Pb and other elements were obtained using X-ray fluorescence (XRF) analysis (Boyle 2000). Sediment samples were analysed using an S2 Ranger XRF spectrometer at the Department of Geography, University of Liverpool.
The chronology for Åbodasjön was based on 25
210Pb dates, three 14C macrofossil dates and four Pb
pollution age markers. The chronology for
Lindhultsgöl was based on 19 210Pb dates, two 14C
macrofossil dates and five Pb pollution age markers (Paper I). To establish chronologies for the two lakes, stratigraphic age-depth models were constructed using the P_Sequence deposition
120 100 80 60 40 20 0 Sediment depth (cm) 2000 1000 Age (AD) 0 100 200 a) Pb (μg g-1) 80 60 40 20 0 Pb (μg g-1) Age (AD/BC) 0 100 200 Sediment depth (cm) 2000 1000 0 1000 b)
Figure 6. Age-depth models based on 210Pb dates (horizontal black bars), calibrated radiocarbon dates obtained from terrestrial
macrofossils (blue) and bulk samples (green), and Pb pollution marker horizons (horizontal red bars), together with the records on total Pb concentration (red) for: a) Åbodasjön and b) Lindhultsgöl. Depths of Pb pollution marker horizons are indicated by the red dashed lines. The modelled ±1σ and ±2σ confidence intervals are indicated by dark and pale grey shading, respectively.
model in OxCal4.1 (Bronk Ramsey 2008), where depths between samples were introduced to constrain the calibration probability intervals. All
ages were expressed as calendar years AD, with a 2σ
confidence interval (95.4% probability). Åbodasjön chronology covers the period from AD 1200 to 2008, and the chronology for Lindhultsgöl covers approximately 2000 years. Figure 6 shows the chronologies for Åbodasjön and Lindhultsgöl. The chronology of the nearby lake Fiolen is described in detail by Fredh et al. (2012).
4.2 Environmental reconstructions
4.2.1 Lake-water chemistry
Two different methods, both based on the same approach using surface-sediment training sets for biological or geochemical indicators, can be used to reconstruct past changes in environmental variables in lake water, such as TOC and pH. Samples are collected from a series of lakes to analyse present-day environmental variables of interest, and these are then compared to the indicators preserved in the surface sediments using statistical techniques. If there is a high degree of certainty, quantitative transfer functions can be developed and used to infer past specific environmental variables.
In this study, VNIRS was used to infer the TOC concentrations of the lake water (Paper III). The use of VNIRS in palaeolimnology was introduced in the 1990s (Korsman et al. 1999) for quantitative inferences of, for example, TOC concentrations, pH and phosphorus concentrations in lake water. The method has since been developed using training sets for northern and southern Sweden (Rosén, 2005; Cunningham et al., 2011), and northern Canada (Rouillard et al. 2011). VNIRS measures the absorbance of wavelengths of 400-2500 nm, reflecting the molecular vibrations of organic compounds in the sediment. Past changes in TOC concentration in lake water were reconstructed using a calibration model based on VNIRS of surface sediments from 140 Swedish lakes covering
a gradient from 0.7 to 24 mg L-1 (Cunningham et
al. 2011). The inferred TOC concentrations in Lindhultsgöl exceeded the range covered by the calibration set, and an additional inference model
from Canada, based on 160 lakes with a DOC
range of 0.6-39.6 mg L-1 was used (Rouillard et al.
2011). The performance of the combined Swedish and Canadian calibration set is similar to that of the
Swedish calibration set, showing an R2 value of 0.6
between measured and predicted TOC concentration in lake water and a root mean square
error of prediction of 4.1 mg L-1 (10.5% of the
gradient).
The pH in the lakes was inferred from sedimentary diatom assemblages using calibration models (Paper III). Diatoms are unicellular, eukaryotic organisms well preserved in lake sediments (Smol and Cumming 2008). Different taxa have different environmental optima, and analysis of assemblages of fossil species can therefore be used to reconstruct environmental variables (Battarbee et al. 1999). Diatom assemblages were separated from the sediment in the laboratory using the water-bath technique described by Renberg
(1990b) with digestion of H2O2 (Battarbee et al.
2001). To estimate diatom concentrations, a known quantity of divinylbenzene microspheres was added to the digested and cleaned samples (Battarbee and Kneen 1982; Wolfe 1997). The samples were evaporated onto cover slips, and at least 400 diatom valves per sample were counted and identified using a light microscope, largely using available reference material, for instance, Krammer and Lange-Bertalot (1986-1991). The pH was inferred using the quantified diatom data in the European Diatom Database combined pH training set online (http:// craticula.ncl.ac.uk/Eddi/jsp/).The calibration set for the model consists of 627 lakes with a pH range of 4.3-8.4. The diatom-inferred pH in the lake water was analysed using a locally weighted average model and inverse deshrinking. In total, 18 samples were included for each lake and the results are presented in Figure 7.
4.2.2 Nutrient cycling and the origin of
organic matter
Organic matter in the lake basin originates from the terrestrial surroundings and aquatic sources, in dissolved and particulate form. Sedimentary organic geochemical records can provide fundamental information on the origin and fate of the organic
matter preserved in lake sediments. In this study, various organic proxies were applied, including bulk organic carbon (TC) and nitrogen (TN) content, atomic C/N ratios (Papers I and III), stable carbon
and nitrogen isotope ratios (δ13C and δ13N) and
hydrocarbon concentrations (n-alkanes) (Paper I). Atomic C/N ratios of lignin- and cellulose-rich terrestrial organic matter are typically >20, whereas aquatic organic matter is characterized by atomic C/N ratios <10 (Meyers and Lallier-Vergès 1999).
Values of δ13C and δ15N for organic matter are
influenced by a number of factors, and they have been used in combination with TC and TN composition of lake sediments to assess changes in
carbon and nitrogen sources and nutrient balance, often in response to anthropogenic activities in watersheds (Meyers and Teranes 2001; Talbot 2001). Hydrocarbon molecules (n-alkanes) are widely used to assess sources of organic matter because of their long residence times and resistance to degradation. Molecular distributions of n-alkanes can be used to differentiate biological sources. Most aquatic algae and photosynthetic bacteria are
dominated by n-C15, C17 and C19 alkanes, while
vascular plants contain large proportions of n-C27,
C29, and C31 alkanes in their epicuticular wax
coatings (Cranwell et al. 1987; Rieley et al. 1991; Tenzer et al. 1999).
Figure 7. Diatom stratigraphy of the major (>5%) taxa, in: Åbodasjön (upper panel) and Lindhultsgöl (lower panel), expressed as relative abundance (%), proportion of planktonic and benthic taxa (%), diatom concentration, diatom accumulation rate and inferred pH. 1300 1400 1500 1600 1700 1800 1900 2000 Y ea r ( A D ) Aulac oseir a amb igua Aulac oseir a dist ans Aulac oseir a sub arctic a/pus illa Aulac oseir a ten ella Othe r Aula cose ira sp p. Disc ostel la ps eudo stellig era Punc ticula ta rad iosa Cyclot ella r ossii Tabe llaria flocc ulosa Achn anthi dium minu tissim um Othe r Ach nanth es sp p. Sum of Eu notia spp. Frag ilaria exigu a Othe r frag ilario id sp p. Frus tulia rhomb oides Navic ula m inisc ula Navic ula rh ynco ceph ala Othe r Nav icula spp. Diatom conc entra tion (va lves g -1 dry s edim ent x 106) Diatom accum ulatio n rate (va lves c m-2 y -1 x1 04) pH 20 20 20 20 20 20 20 20 20 0 50 100 0 500 1000 0 500 1000 6.0 6.5 7.0 Plankton Benthos Sum of pla nkton ic tax a (%) S um of benth ic tax a (%) 1200 1300 1400 1500 1600 1700 1800 1900 2000 Y ea r ( A D ) 20 40 60 Aulac oseir a amb igua 20 Aulac oseir a dist ans Aulac oseir a lac ustris Aulac oseir a lira ta Aulac oseir a niva lis 20 Aulac oseir a pus illa Aulac oseir a sub arctica 20 40 Aulac oseir a ten ella 20 Tabe llaria flocc ulosa Sum of Ac hnan thes s pp. Cymb ella s ilesia ca 20 Euno tia in cisa Stau rosira cons truen s 20 Frag ilaria exigu a Othe r frag ilario id sp p. 20 40 60 Frustul ia rho mboid es 20 Sum of Na vicula spp. Sum of Pin nular ia sp p. Sum of pla nkton ic tax a (%) Su m of benth ic tax a (%) 0 500 1000 Diatom conc entra tion ( valve s g-1 dry se dimen t x10 6) 0 500 Diatom accum ulatio n rate (v alves cm-2 y -1 x10 4) 5 6 7 pH Plankton Benthos 0 50 100 1200
Bulk organic TC and TN contents were analysed during combustion using a Costech ECS 4010 elemental analyser at the Department of Geology,
Lund University. Values of δ13C and δ13N were
analysed using a VG-isotope Micromass dual-inlet mass spectrometer equipped with a EuroVector elemental analyser and a continuous flow inlet, at the Department of Geography and Geology, University of Copenhagen. The data are reported in δ-notation; δ = [Rsample/Rstd - 1] x 1000, where R
denotes the 13C/12C and 15N/14N ratios in the
samples, and VPDB and AIR standards, respectively.
To determine the n-alkanes, sediment samples
were extracted with a mixture of CH2Cl2 and
CH3OH (9:1 v/v). The extracts were reduced using
a Büchi rotovapor and injected in splitless mode into an Agilent 6890 gas chromatograph equipped with a HP5-MS column, at the Department of Geological Sciences, Stockholm University. The chromatograph was interfaced with an Agilent 5973 mass spectrometer. The n-alkane concentrations were normalized with respect to sediment carbon contents.
4.2.3 Catchment-scale land use
Palaeoecological studies based on fossil pollen assemblages found in sediments have been an important tool, for almost a century, in reconstructing past vegetation and anthropogenic activities, such as tree succession and woodland clearance for crop cultivation (Bennett and Willis 2001). It has previously been difficult to estimate past vegetation based on fossil pollen, due to known biases in the way in which vegetation is represented in the fossil pollen record, for example, differences in pollen productivity and dispersal between taxa (Broström et al. 1998; Sugita 2007 a; b). In the present work, fossil pollen records from Åbodasjön, Lindhultsgöl and Fiolen were used to reconstruct and quantify past local land use and associated vegetation using the LRA (Sugita 2007a; b), which compensates for these differences (Papers II, III and IV).
For pollen analysis, sediment samples were treated with the acetolysis method and mounted on slides for identification using a light microscope
(Berglund and Ralska-Jasiewiczowa 1986). At least 1000 pollen grains corresponding to the selected taxa were identified and counted in each 20-year time window, using identification keys (Moore et al. 1991; Punt et al 1976-2009, Beug 2004) and the reference collection at the Department of Geology, Lund University.
For a reliable reconstruction of regional and local vegetation and land use there are two steps using the LRA approach illustrated in Figure 8. The first step in the LRA is to reconstruct the regional plant abundance and composition within a 50-100 km radius using the REVEALS (Regional Estimates of Vegetation Abundance from Large Sites) model. The second step is to apply the LOVE (Local Vegetation Estimates) model for the reconstruction of local vegetation and land use using the background pollen loading based on the regional plant abundance estimates obtained from the REVEALS model. The REVEALS model was applied to pollen counts from Fiolen and Åbodasjön, and the LOVE model was subsequently applied to pollen counts from Lindhultsgöl. Similarly, the REVEALS model was applied to pollen counts from Fiolen and Lindhultsgöl, and the LOVE model was subsequently applied to pollen counts from Åbodasjön. The modelled radii of the relevant source area of pollen (RSAP) for the two lakes, Åbodasjön and Lindhultsgöl, were 1740 and 1440 m from their respective centres (Fig. 4). It is important to note that the model covers a larger area (RSAP) than the actual catchment area, and in this study, changes in land use within the RSAP are assumed to reflect catchment variations in general.
REVEALS LOVE Regional vegetation Local vegetation in the RSAP Pollen productivity and fall speed Pollen data from Fiolen Pollen data from Åbodasjön Pollen data from Lindhultsgöl
Figure 8. Flow chart illustrating how the Landscape Reconstruction Algorithm was used in this study to quantify past regional and local land use.
